The invention relates to a process and apparatus for the capacitive demineralization of liquids containing ions.
In the publication WO 98/17587, prior-art processes and apparatuses for water treatment, in particular demineralization and decarbonization, are described.
These are based on two fundamental process principles, one concerning decarbonization with cation-exchange resins in H+ form and the other concerning decarbonization with precipitation of carbonates, especially lime.
The disadvantage of the known processes, in which the decarbonization is realized by raising the pH, is seen in this document in the fact that large quantities of chemical additives have to be admixed with the water. For example, the addition of lime milk introduces additional Ca2+ ions into the water, which in the subsequent lime precipitation process cannot always be precipitated out completely. On the use of sodium hydroxide solution and soda in the drinking water sector, it is stated that the limit values for sodium ions set corresponding limits here.
A further problem is seen in the high pH of the water after the lime precipitation. Appropriate countermeasures have to be taken for this.
In the case of electrochemical processes as an alternative, it is regarded as a problem that it is important for the electrode reaction to be carried out at a voltage below the water decomposition voltage of 1.23 V, since otherwise H+ ions form on the anode, and they would lower the pH again. As a result, the success of the process is restricted to the oxygen present in the water and its electrochemical conversion at the cathode.
To avoid these problems, it is proposed in WO 98/17587 to form the necessary OHxe2x88x92 ions by electrolytic decomposition of water at the cathode. In this case, a diaphragm is to be used to prevent the OHxe2x88x92 ions from recombining with Hxe2x88x92 ions formed at the cathode.
However, investigations carried out within the scope of the present invention have found that the known process and the corresponding apparatus are relatively complex in terms of control technology and process engineering to the extent that it is necessary to apply a voltage of higher than 1.22 V between the cathode and the anode, and wherein an alkaline environment in which carbonate and/or magnesium carbonate precipitates must be created in the cathode space which is created by introducing a diaphragm between the cathode and the anode.
It is therefore an object of the present invention to provide a process and apparatus for the capacitive demineralization of liquids containing ions which can be realized in a simple manner even on an industrial scale and permit a high level of efficiency.
According to the principles of the present invention, a configuration of highly capacitive electrodes, which preferably consist of a conductive material with an extremely large surface area, is used in a filter container with a filter wall or diaphragm wall. By applying an electric field, in a next process step the ions contained in the liquid are conducted through the filter wall of the filter container and form a so-called bilayer on the surface of the electrode.
After that, a series of pulsed voltage peaks are introduced into this bilayer, representing a supersaturated solution, which leads to the precipitation of microcrystalline structures resulting from ionic compounds. These microcrystalline structures can then be removed from the filter container.
For instance, it is possible to apply either a DC voltage field or an AC voltage field to the highly capacitive electrodes or capacitor elements. A closed cup-shaped filter wall surrounding the highly capacitive electrodes and with a pore size which largely prevents ingress of water but allows the ions having a hydration sheath to pass through is preferably used as the diaphragm wall or filter wall.
For removing the precipitated microcrystalline structures, suitable filtration methods or other retention methods may be used.
Other dissolved ions which cannot be precipitated by feeding the pulsed voltage peaks can be detached, preferably by suitable depolarization or polarity reversal of the capacitor elements or highly capacitive electrodes, and extracted from the medium by bonding on suitable surfaces or by pulsed rinsing operations.
Especially for industrial use, the apparatus according to the invention may preferably be constructed in a modular and meandering form, in order to increase the efficiency and make the polarization reversal possible without major electrical losses.